The invention relates to temperature compensation systems, and relates in particular to temperature compensation systems for linear power amplifiers that include power transistors.
The requirements of a bias circuit for a power transistor are typically quite different from that of a small-signal gain stage. First, the bias circuit should be able to provide sufficient DC base current. Second, linear power amplifiers are usually class AB type for efficiency reasons and this makes achieving constant gain between small and large signal conditions difficult to achieve. This is a requirement for good linear performance. Further, it is typically observed that power amplifier gain exhibits different variation over temperature under small signal and full power operation. A good bias circuit should minimize this effect. A poor bias circuit may result in so much gain drop at high temperature that the power amplifier might be able to deliver only half of its rated power at the extremes of temperature. In general, gain drops with increasing temperature if bias current is kept constant. A gain drop may even be significant for a power amplifier in which the bias current is PTAT (Proportional To Absolute Temperature).
Some of the reasons for these problems are that a PTAT bias current (IC) helps to keep emitter charging time (rE, CJE) constant with temperature, where rE=(kT/qIC). Device gain, however, which is directly proportional to fr, is determined by total emitter to collector transit time, and not only by emitter charging time. The values τB (minority carrier drift-diffusion base time constant), τSCL (collector space charge layer delay time) and τCIB (carrier diffusion time through ‘current-induced-base’ width WCIB) are significant contributors to the emitter-collector transit time at high frequencies and high current densities. When the power amplifier is delivering near full power; these high current density effects kick in and modify the temperature dependence of these device parameters. In other words, self-heating of the power transistor at high currents decreases the saturation velocity in the collector-base depletion region and the electron mobility in the base, which result in increase in τCIB and τB. This is the primary reason for higher rate of gain drop with temperature when the power amplifier is delivering near full power.
Another problem is that, to achieve better efficiency most linear power amplifiers are class AB type and therefore exhibit self-biasing. This means that DC bias, the temperature dependence of which is controllable, is only one part of the total bias. The rest of it comes from the radio frequency (RF) signal itself. If driver-stage gain drops with temperature therefore, it will directly result in reduced gain of the power-stage at elevated temperatures. The DC bias for the power stage should then be able to compensate for this effect.
Certain power amplifiers have employed a variety of approaches to correct for these problems. For example, U.S. Pat. No. 6,369,657 discloses various systems that employ one or more of resistive biasing, active biasing and a current mirror bias network. These solutions primarily result in providing some form of PTAT current through the power transistor, but have been found to not fully compensate for temperature in all applications.
There is a need, therefore, for an improved temperature compensation system for power amplifiers.